Measuring Water Flow Velocity And Hydraulic Parameter Research In Purple Soil

Purple soil is an important soil resource in south China.The soil with a soft,loose structure,easy to weathering,erosion resistance is poorer,the characteristics of soil and water conservation ability weak,and easily formed near the earth’s surface above the top layer aquitard flow,under the effect of coupling surface and near surface water often causes strong soil erosion,at the same time under the condition of current scour will carry a large amount of sediment,aggravate the occurrence of soil and water loss and soil erosion,and water erosion and transport of sediment ability than raindrops hit the erosion and handling forces,combined with slope surface runoff and flow velocity,mutual influence,interaction between again at the same time affected by outside conditions such as flow and slope,makes the intensity of soil erosion in purple soil area and is second only to the north of the loess,the rill erosion is one of the important composition of slope soil erosion,and also is an important process of water erosion of purple soil region slope as well as an important source of small watershed sediment,and the development and evolution of the basin landform process have an important potential impact.Slope runoff velocity is an important parameter affecting slope soil erosion characteristics,and its accurate measurement is very important for soil erosion research and prediction.Therefore,it is of great practical significance to study the influence of runoff erosion on slope velocity under different working conditions.In this study,artificial soil groove was used to simulate the soil erosion process of rill on purple soil slope when concentrated water flow was scoured.The experimental data involved three flow levels:2、4 and 8 L/min and four slope levels:5°,10°,15°and 20°.The flow velocity of rill in purple soil was measured by using electrolyte pulse boundary model method,and the characteristic parameters of soil erosion in rill in purple soil were further determined by the basic data of flow velocity,etc.,and the change rules of runoff pattern,flow pattern and resistance characteristics of slope were analyzed,and the mechanism and dynamic mechanism of soil erosion process in rill in purple soil were expounded.The results show that:（1）Under different gradient and flow conditions of saturated purple soil,the flow velocity of the rill in the initial state varies between 0.17～0.778 m/s,and that of the rill in the stable state varies between 0.156～0.619 m/s.The flow velocity increases with the increase of the slope and flow.The relation of rill flow rate with slope and flow rate can be expressed by linear function.The multivariate linear regression analysis is used to obtain the positive correlation between flow rate and slope convection rate.The influence of slope on flow rate is greater than that of flow rate.The rill flow velocity in the initial state was significantly higher than that in the steady state,with an increase range of 23%～26%.（2）The rill flow velocity of unsaturated purple soil at different near-surface flow depths showed an increasing trend with the increase of slope and flow rate,and the velocity range from 1/4 to 3/4 was 0.215～0.599 m/s,0.198～0.439 m/s,and 0.189～0.559m/s,respectively.Through multiple linear stepwise regression analysis,it can be seen that slope and flow rate have positive correlation effects on rill velocity at different near-surface flow depths,and the influence of slope is greater than that of flow rate.There is no interaction between velocity and slope at 2/4 depth,but there is an interaction between velocity and slope at 1/4 and 3/4depth.Rill velocity of saturated purple soil was lower than that of unsaturated purple soil under the same condition.When near-surface seepage interacts with surface water flow,strong soil erosion will lead to a slow flow rate,especially when the slope is 15°and the flow rate is 8L/min.In this case,the flow rate of 2/4 deep rill<saturated slope≈3/4 deep flow rate<1/4 deep flow rate.（3）Under different working conditions,the average runoff depth increases with the increase of flow and decreases with the increase of slope.The average runoff depth decreases with the increase of near-surface flow depth.The average rill runoff depth of saturated purple soil in initial state is less than that in stable state.The relationship between average runoff depth,flow rate and slope is a power function,which can be fitted by formula h=a Q ^bS ^c.（4）The Reynolds number（Re）varies from 410 to 1670 under different working conditions.The change of Re is mainly related to the flow rate,and increases with the increase of the flow rate.Froud number（Fr）values were all greater than 1 in each working condition,indicating that rill runoff belongs to rapids in the whole rill development stage.Fr increases with the increase of slope,and shows different variation trends with the increase of flow.Fr has no obvious rule with the change of saturation depth and erosion degree.（5）The darcy resistance coefficient（f）of rill varied with the flow rate,slope,saturation degree and erosion degree,but the change rule was not significant.There is a power function relation between f and Re,which can be fitted by the formula f=a Re^-b,and f usually decreases with the increase of Re.